Polymeric foams can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be used as a matrix to contain various liquids and gases. They are capable of various industrial applications such as, for example, use in wipes and diapers, as carriers and ion exchange resins. For some of these applications, it is desirable to have porous crosslinked polymer blocks which have a very low density and a high capacity of absorbing and retaining liquids. Such high absorption capacity, low density, porous polymer blocks can be prepared by polymerizing a specific type of water-in-oil emulsion known as high internal phase emulsion (HIPE) having relatively small amounts of a continuous oil phase and relatively greater amounts of an internal water phase.
Such high absorption capacity, low density foams are prepared in U.S. Pat. No. 4,522,953 by polymerizing and crosslinking the monomers in the continuous oil phase of a high internal phase water-in-oil emulsion with a polymerization initiator such as potassium persulfate. Generally, these high internal phase water-in-oil emulsions contain at least 90 weight percent of an aqueous liquid as the internal phase. The high ratio water-in-oil emulsions are formed by combining the oil phase with water under moderate shear. In order to obtain this high internal phase water-in-oil emulsion, a surfactant must be used to stabilize the emulsion.
One class of surfactants used to produce foams by such processes are sorbitan fatty acid esters. Commercial sorbitan fatty acid esters are a combination of mono-, di-, tri-, and tetra-fatty acid esters of sorbitan C.sub.6 (H.sub.2 O).sub.5 H.sub.2, as well as mono- and di-fatty acid esters of isosorbide C.sub.6 (H.sub.2 O).sub.4 H.sub.2 and polyol impurities. Commercial sorbitan fatty acid ester surfactants form an undesirable water soluble sludge when these surfactants are dissolved in an oil phase. It has been found that polyols such as sorbitan and isosorbide are the main components of the sludge. The sludge must be removed before the surfactant in the oil phase is used to prepare an emulsion in order to prevent plugging of the mixing equipment. Thus, it is advantageous to reduce the sludge concentration in the oil phase containing sorbitan fatty acid ester surfactants.
The stability of the emulsion is directly tied to the surface activity of the emulsifier. A robust emulsifier will stabilize emulsions up to high (30:1) water:oil ratios. A poor emulsifier will not form stable emulsions, and at high water to oil ratios the emulsion will degrade by not incorporating water, and eventually break completely.
One method of enhancing the emulsifier is to combine different sorbitan fatty acid esters as reported in U.S. Pat. No. 5,200,433. However, it is desirable to further enhance the emulsion stability to obtain higher ratio water-in-oil emulsion increasing the overall absorption capacity.
Further, relatively large amounts of sorbitan fatty acid ester surfactants are required to prepare the high internal phase emulsions. To stabilize an emulsion consisting of 30 parts aqueous phase dispersed in 1 part oil phase by volume, a 10 percent by weight or greater concentration of sorbitan fatty acid ester surfactant in the oil phase is generally needed. The use of such a large amount of surfactant is undesirable since it increases the raw material cost of producing a foam. Thus, it is desirable to be able to form a stable emulsion with less surfactants.
Further, some sorbitan fatty acid ester surfactants, such as sorbitan monolaurate, are not effective emulsifiers at elevated temperatures. However, it is desirable to be able to use these surfactants to prepare high internal phase emulsions at elevated temperatures.
It is therefore an object of the present invention to provide a more effective surfactant system for the preparation of a high internal phase water-in-oil emulsion useful for preparing low density crosslinked polymeric materials.